This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In the absence of an effective vaccine, other methods for preventing the sexual transmission of human immunodeficiency virus type 1 (HIV-1) must be pursued. To guide vaccine design, we assessed whether human monoclonal antibodies (MAbs) b12 and b6 against the CD4 binding site on HIV-1 gp120 and F240 against an immundominant epitope on gp41 could prevent vaginal transmission of SHIVSF162P4 to macaques. The two anti-gp120 MAbs have similar monomeric gp120-binding properties, measured in vitro, but b12 is strongly neutralizing while b6 is not. F240 is non-neutralizing. Applied vaginally at a high dose, the strongly neutralizing MAb b12 provided sterilizing immunity in 7/7 animals, b6 in 0/5 animals and F240 in 2/5 animals. Compared to control animals, the protection by b12 achieved statistical significance whereas that due to F240 did not. Additional passive transfer experiments also indicated that the ability of the administered anti-gp120 MAbs to neutralize the challenge virus was a critical influence on protection. Furthermore, there was a significant increase in the number of founder viruses establishing infection in animals receiving MAb b6, compared to other non-protected macaques. Thus a gp120-binding, non-neutralizing MAb against gp120 was, at best, completely ineffective at protection. Non-neutralizing antibodies to gp41 may have a limited capacity to protect, but the results suggest that the central focus of HIV-1 vaccine research should be on the induction of virus-neutralizing antibodies.